The present invention relates to handling of paper or cardboard webs formed in a paper or cardboard machine, especially to leading the web during the subsequent handling process after the last nip of the press section of the machine.
In a paper or board making machine, the web is formed by feeding a slurry of fibrous material from a head box onto a wire. Water is removed from the web on the wire at various pressing stages. At the beginning of the formation of the web, its strength is extremely low since the web contains only small amount of fibers compared to the amount of water. In the press section, the solids content of the web increases rapidly, but the web is still quite wet and its strength low. After the press section, the web is dried by different dryers using heat. The dried web is then normally treated in different processes to improve its properties according to a desired use of the web. The most common finishing processes are calendering, sizing and coating.
On the press section of the machine, the web is supported by a wire or felt, but in subsequent drying stages and during finishing, the web is guided by rolls or, in modern machines, also by belts, felts or wires. When the web is transferred forwards in the manufacturing process, it has to be supported on different surfaces, especially when the web is coated. The path of the web comprises usually several open draws wherein the web travels unsupported between two supporting members through open air. Since the web has to be tensioned to keep it on the supporting members and to compensate for the changes in web length and width during processing, a speed difference has to be set over the open draws on the supporting and driving members guiding the web. These open draws are necessary for a number of reasons. First, the shrinking, elongation and widening of the web has to be compensated as stated above. The largest dimensional changes of the web are caused by moisture content variations during processing and, if the variations are big, open draws are necessary since the dimensional changes cannot be compensated when the web is supported on a roll, belt or similar dimensionally rigid member. Second, it is difficult to remove a wet web from a support member and transfer it to a following member without special measures, since the web usually tends to follow the surface on which it is already attached to. It is often easier to remove the web from one support member and transfer it over an open draw to the next support member. Third, some of the apparatuses used for paper and cardboard making require that the web is led along a defined path through the process whereby it may be difficult to lead the web supportedly to the apparatus and from it. In some apparatuses, like roll calenders or coaters, the web cannot be supported continuously without changing the characteristic of the process. In calenders, the performance of the nip changes if, for example, a support belt is lead through the roll nips. In coating, the wet coated side of the web cannot be touched, whereby the web has to be supported on the dry side of the web, and there has to be an open draw between successive support members. The length of the open draw is determined by diameters of the rolls guiding the support member.
Even though the open draws are often necessary, they cause several problems. After the web is formed in the press section, it is still very wet and breaks easily. When such a web is taken over an open draw, the speed differences of the web transferring elements are supposed to keep the web on a predetermined tension so that no slacking or wrinkling of the web occurs. However, the speeds of the wires, rolls, belts and other elements transferring the web may vary slightly, whereby the tensions over the open draws changes. The tensile strength of the web may also vary, for example, because of variations in the moisture content. Further, there may be defects in the web. These and other factors may lead to a situation where the tension of the web exceeds the strength of the web, first usually on a limited area of the web, whereby the web breaks. Web breaks are the main reason for production stops.
One of the factors that influence heavily on the quality of the product and production costs in paper and board manufacture is drying of the web. As stated above, a web is first formed by a head box, wire and press section into a reasonably solid web, and the web is then dried to a desired moisture content, which is about 2% to 4% of the weight of the web. If the web is calendered, it has to be wetted thereafter to a higher moisture content depending on the calender type, and if the web is coated or sized, the web absorbs water from the coating or sizing mixture. The water added to the web on calendering evaporates normally during the calendering process, but the water absorbed in the web during coating or sizing has to be dried. The drying process requires a lot of energy and the dryers are quite costly and especially on fast machines very long drying sections have to be used, whereby the drying of the web after formation and during finishing forms a notable part of the manufacture costs and the cost of the machine.
One possibility to decrease the investment costs of the drying and dryers on a paper or board manufacturing machine is to combine coating and sizing with the drying of the web on an on-line manufacturing line so that the treating agent is spread on the web in the drying section of the manufacturing line. The problems of this solution are the mechanical stresses imposed on a wet web on application of the treating agent and increased moisture of the wet web that requires efficient drying process.
According to the present invention the web is transferred continuously supported through a drying section of a paper or cardboard machine, and both sides of the web are treated with a treating agent on a long nip applicator included to the drying section of the machine.
According to other aspects of the present invention, the web is transferred from the last press nip of the press section to the first dryer of the dryer section with a continuous loop-like support member, for example, by a transfer belt, transfer felt or one of the press felts of the press section.
The web is advantageously dried in the drying section with a blow unit on a porous drying felt by blowing hot air on the web with impingement dryers and the drying felt may be cooled.
Other objects and features of the invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.